A transparent conductive film has high conductivity and high light transmittance in the visible light region, so is used as a transparent electrode of various kinds of photoelectric conversion elements including solar cells, liquid-crystal display elements, and various other kinds of light receiving elements. Moreover, transparent conductive film has excellent reflection absorption characteristics in the near-infrared ray region, so transparent conductive film is also used as heat-ray reflection film, various kinds of antistatic film, transparent heating elements for defogging freezer showcases, and the like.
As material for this kind of transparent conductive film, typically tin oxide (SnO2) that includes antimony, fluorine or the like as a dopant, zinc oxide (ZnO) that includes aluminum, gallium, indium, tin or the like as a dopant, indium oxide (In2O3) that includes tin, tungsten, titanium or the like as a dopant, and the like are used. Particularly, indium oxide film that includes tin as a dopant (ITO) is such that a transparent conductive film having low resistance can be easily obtained, so is widely used industrially.
As a method for producing this kind of transparent conductive oxide film, a sputtering method, a vapor deposition method, an ion plating method, a coating method using a coating solution for forming a transparent conductive film, and the like are used. Particularly, the sputtering method and ion plating method are useful in the case of forming a film on a substance such as a substrate on which the film is to be formed using a material low in vapor pressure, or for uses when precise film thickness control is necessary, and is also widely used because operation is extremely convenient.
The sputtering method is a method in which typically argon plasma is generated in an argon gas atmosphere at a pressure of approximately 10 Pa or less by causing a glow discharge between a substrate as an anode and a target as a cathode, then causing collisions between the argon cations in the plasma and the target, causing the particles of the target component be sputtered off and the sputtered particles to be deposited on the substrate to form a film.
The sputtering method is classified by the method for generating argon plasma, and there is a high-frequency sputtering method that uses high-frequency plasma, and a direct-current sputtering method that uses direct-current plasma. Moreover, a magnetron sputtering method is also used in which a film is formed by arranging a magnet unit on the back side of the target, causing argon plasma to be concentrated directly above the target, and increasing the collision efficiency of argon ions even at low gas pressure.
Of these, normally a direct-current magnetron sputtering method is used for producing a transparent conductive oxide film. Moreover, a high-frequency superimposed direct-current sputtering method that uses a direct-current plasma as a base may be used. This high-frequency superimposed direct-current sputtering method makes it possible to perform sputtering at a low discharge voltage, which makes it possible to obtain a good quality film by reducing collisions on the film due to oxygen ions generated from the target, so this method is used when producing an oxide film using an oxide target.
Furthermore, film formation by the sputtering method can be largely divided into a stationary facing target type sputtering method, and an inline-type sputtering method. The stationary facing target type sputtering method is a method in which inside a vacuum film formation chamber, a substrate moves to a position directly above a target and stops, then plasma is generated over the target and film formation is performed for a set about of time, and at the instant that a film having a specified film thickness is formed, electric discharge is stopped, and the substrate is moved away from the target.
On the other hand, the inline-type sputtering method is a method for forming a film on a substrate by continuously generating and maintaining argon plasma over a target in a vacuum film formation chamber, conveying the substrate closer to the target at a constant moving speed, and passing the target without stopping. In this case, the film thickness of the thin film that is obtained is controlled by the electric power inputted to the target and by the conveying speed. This kind of inline-type sputtering method is capable of uniformly forming a film over a large surface area, so it is the most widely used industrially.
This kind of transparent conductive oxide film is widely used as a transparent electrode of a photoelectric conversion element such as that of a solar cell and the like. Solar cells that are a kind of photoelectric conversion element are configured having a layered structure of p-type and n-type semiconductors, and are largely divided according to the type of semiconductor that is used. Normally, safe and richly abundant silicon is used as the semiconductor material for a solar cell. As silicon type solar cells there are single-crystal silicon solar cells, polycrystalline silicon solar cells, amorphous thin-film silicon solar cells, hybrid solar cells that are a combination of amorphous thin-film silicon and single-crystal silicon, and the like. Moreover, development of compound thin-film solar cells that use a thin film of a compound semiconductor such as CuInSe2, GaAs, CdTe and the like as a semiconductor material is being performed.
Amorphous thin-film silicon solar cells, hybrid solar cells, and compound thin-film solar cells are such that the use of transparent conductive oxide film as an electrode on the side solar light enters into the solar cell is necessary. As this kind of transparent conductive oxide film, ITO film, and aluminum or gallium-doped ZnO film are used (refer to JPH06338624 (A), and JP2003115599 (A)).
The transparent conductive oxide film that is used as a transparent electrode for a solar cell is required to have characteristics such as low resistance and high transmittance of solar light. The spectrum of solar light includes from 350 nm ultraviolet rays to 2500 nm infrared rays, and so as to be able to effectively convert this light energy to electric energy, a transparent conductive oxide film that is able to transmit light in as wide a wavelength range as possible is required. As a transparent conductive oxide film having such characteristics, JP2004207221(A), for example, discloses a transparent conductive oxide film that, is made using a titanium-containing indium oxide (ITiO) of which part of the indium is replaced with titanium, has high transmittance in a wide range from the visible-light region to the near-infrared ray region, and has excellent conductivity.
In the technology disclosed in JP2004207221(A), the transparent conductive oxide film is formed using a sputtering method or an ion plating method. Of these, when using a sputtering method, an ITiO sintered body target is used as the raw material sputtering target, and using a stationary facing target sputtering method in which the substrate and target are placed inside a sputtering apparatus, and in an argon inert gas atmosphere that includes oxygen gas, the substrate is heated to a specified temperature, an electric field is applied between the substrate and the target to generate plasma between the target and substrate, and an ITiO transparent conductive oxide film is formed on the substrate.
On the other hand, in the case of using an ion-plating method, an ITiO sintered body tablet is used as a raw material ion-plating tablet, the substrate is placed inside the ion-plating apparatus, the tablet is placed inside a copper hearth inside that ion-plating apparatus, and in an argon inert gas atmosphere that includes oxygen gas, the substrate is heated to a specified temperature, the tablet is evaporated from the copper hearth using an electron gun, plasma is generated near the substrate, and by ionizing the tablet vapor, an ITiO transparent conductive oxide film is formed on the substrate.
However, with these methods, when an ITiO film is formed on a substrate that will become a photoelectric conversion layer of a photoelectric conversion element such as a solar cell, depending on the film formation conditions, the characteristics of the photoelectric conversion element may not be sufficiently exhibited. Particularly, in the case of forming an ITiO film using an inline-type sputtering method that is widely used industrially, the characteristics of the ITiO film are not utilized, and it becomes remarkably evident that the characteristics of the photoelectric conversion element become insufficient.